Televisions contain a number of components, including a screen such as a cathode ray tube (CRT), liquid crystal display (LCD) or plasma display, as well as circuitry to receive a signal to be displayed and display the signal on the screen. This circuitry includes a tuner that tunes to a particular carrier (frequency) to receive the desired signal.
Television tuners receive carriers over a predetermined range, e.g. from 50 MHz to over 860 MHz for some standards. This is a relatively wide range of frequencies compared to other electronic devices, such as cellular telephones, which operate over a frequency range of 100 MHz or so. The incoming signal power to a tuner can vary by many orders of magnitude depending on a variety of factors such as distance of the tuner to the signal source or the environmental conditions, for example. Moreover, the signal strength may vary continuously, if the television is mobile, as the television is being transported.
However, the change in signal strength is problematic. Before the signal is provided to an output of the tuner, it is amplified by a gain stage having a constant (fixed) gain. In this case, the signal strength provided to the gain stage is adjusted so that the signal is within the range of the gain stage. That is, if the amplitude of the signal is too large, the gain stage will distort the peaks of the signal and thus degrade the output of the tuner. Accordingly, there exists a need to continuously control the gain of the tuner over a wide range of input analog voltages such that a constant output power level is achieved.
To control the gain of the tuner, an amplifier containing the gain stage and a variable attenuator is provided. The attenuator is disposed between the input of the amplifier and the gain stage. It is desirable to have a continuous gain control of the attenuator using an analog control voltage. However, providing variability adds complexity and distortion problems. For example, a tradeoff exists between the sensitivity of the attenuation control voltage and the attenuation range.
In addition, altering the gain of the tuner changes the noise figure of the tuner. The noise figure is the ratio of the total output noise power to the output noise due to a standard input source resistance, e.g. a 50 ohm resistance. In particular, the noise figure of an amplifier having a fixed output level degrades with decreasing gain (i.e. for larger input levels). The noise of the tuner increases when the noise figure reaches a particular value dependent on the configuration of the tuner, primarily as the noise contribution of the gain stage becomes significant.